Effects of Grain Refinement on Tensile Properties and Precipitation Kinetics of Al-Si-Mg Alloys Cast in Sand Molds
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d casting (SMC) is widely used in the production of large-size aluminum alloy castings. However, its lower cooling rate tends to develop coarsening dendrites compared with permanent mold casting.[1] Therefore, many efforts have been made to improve the mechanical properties of aluminum alloys in SMC, but the effects in sand molds are less valid than those in the permanent mold.[2] As we know, the Al-Ti-B refiner is usually used in casting aluminum alloys to refine grains.[3] However, since the formation of Ti-Si compounds consumes the effective content of Ti elements, the Al-5Ti-1B refiner has poor applicability in Al-Si alloys.[4,5] Besides, due to the co-poisoning effect between B and Sr (a commonly used modification element for eutectic Si), the practical application of refiners containing B elements is limited[6–8] LaB6 has been reported as a potential candidate for an effective refiner of commercial pure aluminum because of its
FEIYANG CHEN, TAO LU, and YE PAN are with the Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing, 211189 China. Contact e-mail: [email protected] Manuscript submitted on March 17, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
heterogeneous nucleation.[9,10] Pan et al. also proposed that co-alloying of La and B elements can greatly improve the refinement of a-Al grains in Sr-modified hypoeutectic aluminum alloys in the permanent mold condition.[11] Hence, the addition of La could avoid the formation of undesired Sr-B compounds during the solidification.[8,12] Since the advantages of La–B coalloying on grain refinement have been proven in permanent mold casting, it can be predicted that this result is also true in SMC. Another aspect, the refinement of the microstructure, benefits the distribution of the solution elements, such as Mg and/or Cu, during the solidification because of the limitation of chemical segregation.[13] Also, it leads to the formation of more nucleation sites for precipitates.[14] This effect may lead to a different precipitation kinetics of strengthening phases during the heat treatment, especially the agehardening response.[15–17] Recent articles reported that in permanent molds with high cooling rates, grain refinement altered the precipitation kinetics of the h¢Al2Cu phase in Al-Si-Cu-Mg alloys and also improved the mechanical properties after modified heat treatment.[18,19] Hence, the La–B co-refiner was applied to SMC AlSi-Mg alloys, and its effects on the properties and aging hardening of Al-Si-Mg alloys are discussed. The variations of microstructures and improvement of tensile strength and elongation of the alloys were investigated in the article. The precipitation kinetics of Mg2Si and aging treatment of La–B co-refined Al-Si-Mg alloys were analyzed in detail, so better comprehensive properties of Al-Si-Mg alloys were obtained. In general, the La–B co-refiner is valuable for improving the properties of aluminum alloys cast in sand molds. A356 and A359 alloys were commercial alloys used in SMC and prepar
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